According to an electrodeposition coating method, a coated film can be formed even on narrow portions of a substrate to be coated having intricate shape, automatically and continuously. Therefore, the electrodeposition coating method is widely used for primer-coating a substrate having intricate shape and being required to have high rust resistance, such as an automobile body.
Further, an electrodeposition coating method is superior in utilization efficiency of a coating composition to the other coating method, and it has conventionally been conducted as an industrial coating method due to its economical advantage. The cationic electrodeposition coating method is conducted by dipping a substrate to be coated in an cationic electrodeposition coating composition, in which a voltage is applied with using the substrate as a cathode.
In order to improve corrosion resistance of an electrodeposition coated film, various metal catalysts including lead which act as an anti-corrosion agent have been added to the electrodeposition coating composition. However, it has been required in these days to cut down content of the metal catalyst employed in an electrodeposition coating composition because metal ion, specifically lead ion exerts a harmful effect on the environment.
On the other hand, in proportion as a concern for environmental problems has been grown, harmful air pollutants (HAPs) has been regulated in quantity more tightly over developed countries. An electrodeposition coating composition contains a volatile organic solvent to some extent as a solvent for synthesizing a resin, a flow aid for an electrodeposition coated film, a conditioning agent for coating operation, and the like. Therefore, an electrodeposition coating composition which contains HAPs in a substantial amount, may hardly be used if the environmental regulation is intensified.
It is also desired that consumption of a coating composition itself is reduced, in order to save cost for conducting an electrodeposition coating method.
Deposition of a coating solid which occurs in the course of electrodeposition coating is due to an electrochemical reaction. A coated film is deposited on a surface of a substrate to be coated by a voltage being applied to an electrodeposition coating composition. The substrate is electrically insulated when a coated film is deposited thereon, and electric resistance becomes large as the deposited film becomes thick.
As a result, deposition decreases at the portion on which a coated film has been formed. Alternatively deposition increases at the portion on which no coated film has been formed. Thus, a coated film sticks to an uncoated portion of the substrate, thereby coating process is completed. As described above, a coated film is sequentially formed on the uncoated portion of the substrate during the electrodeposition coating process. Such deposition property of the electrodeposition coating composition is referred to as “throwing power” throughout the specification. An electrodeposition coating composition having good throwing power can form a coated film which has even thickness over a coated surface.
Theoretically speaking, an insulative coated film is formed on a coated surface of the substrate sequentially on the electrodeposition coating process. Therefore, throwing power must be infinity and a coated film be made uniformly over the coated surface. However in fact, since an uncoated portion of the substrate is weak in voltage to be applied, the coating solid hardly sticks to that portion. Therefore, throwing power of an electrodeposition coating composition has not been sufficient, and unevenness of film thickness may have been occurred.
An electrodeposition coated film is usually employed as a primer coating which aims at preventing corrosion or rust from generating on a substrate to be coated. Therefore, even if the substrate is complex in structure, the coated film must have not less than a certain value of thickness at the whole portion. Thus, when unevenness of film thickness is present, the thicker portion is being overcoated, and it means that the coating composition has been consumed excessively. Therefore, in order to increase utilization efficiency of a coating composition, throwing power have to be improved.